1. Field of the Invention
The invention relates to electrically conductive coaxial cable. More particularly, the invention relates to coaxial cables having low electrical resistance, high tensile strength, good flexibility, and which are capable of relatively high frequency signal transmission.
2. State of the Art
Wire is manufactured from ingots using a rolling mill and a drawing bench. The preliminary treatment of the material to be manufactured into wire is done in the rolling mill where white hot billets (square section ingots) are rolled to round wire rod. The action of atmospheric oxygen causes a coating of mill scale to form on the hot surface of the rod which must be removed. This descaling can be done by various mechanical methods (e.g., shot-blasting) or by pickling, i.e., immersion of the wire rod in a bath of dilute sulfuric or hydrochloric acid. After pickling, the wire rod may additionally undergo a jolting treatment which dislodges the scale loosened by the acid. The remaining acid is removed by immersion of the wire rod in lime water.
The actual process of forming the wire is called drawing and is carried out on the metal in a cold state with a drawing bench. Prior art FIG. 1 shows a simple drawing bench 10. The wire 12 is pulled through a draw plate 14 which is provided with a number of holes, e.g. 16, of various diameters. These holes taper from the diameter of the wire 12 that enters the hole to the smaller diameter of the wire 12' that emerges from the hole. The thick wire rod 12 is coiled on a vertical spool 18 called a swift and is pulled through the die by a rotating drum 20 mounted on a vertical shaft 22 which is driven by bevel gearing 24. The drum can be disconnected from the drive by means of a clutch 26. To pass a wire through a hole, the end of the wire is sharpened to a point and threaded through the hole. It is seized by a gripping device and rapidly pulled through the hole. This is assisted by lubrication of the wire. Each passage through a hole reduces the diameter of the wire by a certain amount. By successively passing the wire through holes of smaller and smaller diameter, thinner and thinner wire is obtained.
In the modern wire industry, instead of a draw plate, dies are used. Dies are precision-made tools, usually made of tungsten carbide for larger sizes or diamond for smaller sizes. The die design and fabrication is relatively complex and dies may be made of a variety of materials including single crystal natural or synthetic diamond, polycrystalline diamond or a mix of tungsten and cobalt powder mixed together and cold pressed into the carbide nib shape.
A cross section of a die is shown in prior art FIG. 2. Generally, the dies used for drawing wire have an outer steel casing 30 and an inner nib 32 which, as mentioned above, may be made of carbide or diamond or the like. The die has a large diameter entrance 34, known as the bell, which is shaped so that wire entering the die will draw lubricant with it. The shape of the bell causes the hydrostatic pressure to increase and promotes the flow of lubricant into the die. The region 36 of the die where the actual reduction in diameter occurs is called the approach angle. In the design of dies, the approach angle is an important parameter. The region 38 following the approach angle is called the bearing region. The bearing region does not cause diametric reduction, but does produce a frictional drag on the wire. The chief function of the bearing region 38 is to permit the conical approach surface 36 to be refinished (to remove surface damage due to die wear) without changing the die exit. The last region 40 of the die is called the back relief. The back relief allows the metal wire to expand slightly as the wire leaves the die. It also minimizes the possibility of abrasion taking place if the drawing stops or if the die is out of alignment with the path of the wire.
Although wire drawing appears to be a simple metalworking process, those skilled in the art will appreciate that many different parameters affect the physical quality of the drawn wire. Among these parameters, draw stress and flow stress play an important role. If these parameters are not carefully considered, the drawn wire may have reduced tensile strength. A discussion of the practical aspects of wire drawing can be found in Wright, Roger N., "Mechanical Analysis and Die Design", Wire Journal, October 1979, the complete disclosure of which is hereby incorporated by reference herein.
The wire forming processes described above may be used to form different kinds of wires including wires which are used to conduct electricity and wires which are used as structural supports. Generally, the most important physical characteristic of a wire used to conduct electricity is its electrical resistance. In addition, where wire is used as a signal transmission medium, the attenuation frequency characteristics of a wire are extremely important. In all types of wires, flexibility may also be an important characteristic, with increased flexibility facilitating the snaking of wire through a tortuous path.
Cables are a bundle of wire strands held together, and typically include wire strands twisted together into a rope. Generally, a cable exhibits much more flexibility than a single wire of comparable diameter. Thus, in both structural and electrical applications, where flexibility is important, stranded cables are generally used rather than single solid wires. Stranded cables also have the advantage that they do not kink as easily as solid wires and they can be connected to terminals by crimping. However, stranded cables have some disadvantages, including lower tensile strength and higher electrical resistance than solid wires of comparable diameter. In addition, the rough outer surface presented by stranded cables makes them more difficult to insulate than solid wires.
Prior art FIGS. 3 and 4 schematically illustrate an electrical transmission cable 50, in which several strands of wire are twined to produce a flexible cable having an overall diameter D, but which has a smaller cross sectional area than a solid wire with the same diameter. The cable 50 is shown consisting of seven wire strands 52, 54, 56, 58, 60, 62, 64 each having a diameter "d". In actual practice, an electrical transmission cable may consist of many more conductive strands and one or more steel core strands which serve to enhance the tensile strength of the cable. As shown, the seven strands are twined to form the conductive cable 50 having an overall diameter "D" which is approximately 2.15 d. However, the cross sectional area of the conductive cable 50, for purposes of computing the resistance (or conductance) of the cable is not as large as the cross sectional area of a solid wire having a diameter of 2.15 d. Thus, the stranded and twined cable 50 will have a higher resistance than a solid single strand of wire with the same cross sectional diameter.
Coaxial cable is another type of cable, and is suitable as a signal transmission medium. Coaxial cable generally consists of an unbalanced pair of conductors, in which an inner conductor is surrounded by an outer conductor (shielding layer), and the two conductors are held in a concentric relationship by a dielectric (insulator). The inner conductor is typically a single strand of drawn wire, while the outer conductor is typically a tubular braid of individually drawn wires or a conductive foil. The dielectric can be many different types including polyethylene, polyvinyl chloride, gas injected foams (e.g., nitrogen gas-injected foam polyethylene), other foams, Spirafil.RTM., and air or another gas. Where the dielectric is air or another gas, the inner conductor is maintained in position by the use of discrete spacers. For long-distance telecommunication signal transmissions, coaxial cables are provided in two standard gauges. Small gauge cable includes an inner conductor having an outer diameter of approximately 0.047 inches, and an outer conductor having an outer diameter of 0.174 inches. Large gauge cable has an inner conductor having an outer diameter of approximately 0.104 inches and an outer conductor having an outer diameter of approximately 0.375 inches. The use of a solid wire inner conductor having a diameter of 0.047 inches or 0.104 inches limits the flexibility of the standard coaxial cables. However, unlike electrical transmission lines, a stranded cable is typically not suitable for the central conductor due to standard connectors adapted for terminating free ends of the cable. In addition, when carrying transmission signals, and particularly high frequency signals, coaxial cables tend to generate heat, which is undesirable.
The expansion of the telecommunications industry has created a greater need for coaxial cables exhibiting good signal transmission characteristics, particularly at higher frequencies. In addition, such cables should be highly flexible and preferably capable of dissipating heat.